fix: correctly parse standalone prerelease PR body

[lukekarrys]

Thank you for opening a Pull Request! Before submitting your PR, there are a few things you can do to make sure it goes smoothly:

• Make sure to open an issue as a bug/issue before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea
• Ensure the tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

Fixes #<issue_number_goes_here> 🦕

---

This fixes pull request body parsing for single prereleases. Previously the regex used to match the title would include the rest of the title link as well. This PR changes the regex to match everything up until the next ].

Here's the output of the new test without this change:

AssertionError: expected '3.2.7-pre.0](https://www.github.com/g…' to deeply equal '3.2.7-pre.0'
+ expected - actual

-3.2.7-pre.0](https://www.github.com/googleapis/java-asset/compare/v3.2.6...v3.2.7-pre.0) (2021-10-20)
+3.2.7-pre.0

[chingor13]

Thanks!